1. Field of the Invention
This invention relates to fall board structures for keyboard instruments such as musical instruments having keyboards.
This application is based on Patent Application No. Hei 9-362483 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
According to the conventional fall board structure (or cover structure) employed in musical instruments such as keyboard instruments, an oil damper is used to provide resistance in a cover-close mode to close the fall board (or cover) of the keyboard instrument. In this structure, a valve member and orifice are provided inside of a damper mechanism to increase or decrease a flow (or discharge) of oil for controlling damper force. At a cover-open mode, the valve member opens to increase the flow of oil so that the fall board smoothly opens. At a cover-close mode, the valve member closes to let the oil flow through the orifice. So, the flow of oil is decreased such that the damper force is strengthened. Thus, it is possible to provide strong resistance of the damper only at the cover-close mode. In contrast, it is possible to almost cancel the resistance at the cover-open mode.
In addition, an open/close device uses a torque shaft bush to impart resistance to an open/close member (e.g., cover) at cover-open/close modes, which is disclosed by the paper of Japanese Patent Application, Publication No. Hei 1-137294, for example. According the structure of the open/close device, there are provided two friction mechanisms. One friction mechanism imparts frictional resistance to the open/close member with respect to an overall angle range in rotation (or rotary movement) of the open/close member. Another frictional mechanism imparts frictional force to the open/close member with respect to a specific angle range in rotation of the open/close member, for example, with respect to an intermediate angle range in open/close strokes. Thus, it is possible to avoid rapid open/close movements of the open/close member.
The conventional fall board structure of the musical instrument using the oil damper suffers from a problem due to a risk that occurs in response to a manner to close the fall board. Suppose an event that a user (i.e., a human operator of the musical instrument) performs a cover-close operation just after a cover-open operation. In that event, the valve member is initially placed in an open state, while in a transition from the open state to a close state of the valve member, it is impossible to obtain damper force. For this reason, resistance force does not occur immediately in response to the cover-close operation. Such a phenomenon is expressed by "play" or "hysteresis". This phenomenon causes problems in the case where after the fall board is fully closed, the user slightly lifts up and then releases the fall board, or in the case where the user misses his or her hand to slip the fall board. In those cases, occurrence of the resistance force delays so that the fall board speedily closes by its own weight. Thus, there is a problem that if a hand (or hands) of the user or another person is placed between a base rod portion of the keyboard and the fall board, the hand(s) may be strongly sandwiched between them.
As for the overall process of the cover-open operation and cover-close operation, response or follow-up performance of the resistance force is not so good due to existence of a relatively large "play" of the conventional fall board structure, which lacks comfortableness in operation.
The conventional fall board structure of the musical instrument using the torque shaft bush have a certain degree of freedom in operations because it is capable of imparting "strong" resistance force to the fall board with respect to the specific open/close angle range of the fall board. In addition, it does not have the foregoing problem due to the "play" so much. However, the above fall board structure requires complicated construction using two friction mechanisms, which is a troublesome.